1. South Africa: Adam's Calendar

Easily the oldest human-made structure ever discovered, the Neolithic site named Adam's Calendar appears to be just that: a functioning calendar. It's estimated to be anywhere from 75,000 to 300,000 years old. The calendar's stones are aligned with North, South, East, West, and the winter and summer solstices. There also seems to be a connection between its layout and the stars in the belt of the constellation Orion. Some believe that Adam's Calendar is the missing link to a lost advanced society, but there is considerable debate about how advanced its builders were and whether or not it was just a collection of rocks placed by primitive people.

2. Peru: Machu Picchu

Not all ruins date back to the BC era. Some are more modern, but reflect a civilization that we still don't know much about.

The Incan emperor Pachacuti built the impressive mountainside village of Machu Picchu in the 15th century. The village is halfway up the Andes mountains on a plateau 8,000 feet above sea level. Its startlingly precise construction still isn't fully understood.

Another mystery surrounds the reasons the site was abandoned. Some believe a smallpox outbreak decimated the Incan population. The village was largely ignored until being rediscovered in 1911.

3. Malta: Megalithic Temples

The Mediterranean island nation of Malta is one of the smallest and most densely-populated countries in the world. Also, it boasts still-standing evidence of Neolithic human habitation. Its megalithic temples were once considered to be the oldest extant human structures - dating to 3,000 BC. The island features six separate temple sites and 16 additional ruin areas. Malta is set between the heel of Italy and North Africa, making it a critical historical link between European and African civilizations.

4. Scotland: Knap of Howar

Much farther north, on the Papa Westray island in Orkney, Scotland, you'll find what may be the oldest standing structure in northern Europe. A simple neolithic farmhouse, the Knap of Howar has been carbon-dated to between 3,700-2,800 BC. Inside, there is intact stone furniture which gives archaeologists indispensable insights into how its ancient occupants lived. The construction methods of these prehistoric builders seem pretty clear, as markings on nearby seaside rock quarries show evidence of how thin slabs were cut.

5. Ireland: Newgrange

In nearby Ireland, an impressive building also dates back to the third millennium BC. The Newgrange Tomb is a flat, round stone structure that covers about an acre. Distinctive megalithic art adorns its entrance, including triskellion spirals that profoundly influenced Celtic art and culture.

6. Turkey: Catalhoyuk and Cappadocia

One of the oldest confirmed neolithic sites, Catalhoyuk in central Turkey, dates between 7,000 to 5,700 BC. This proto-city appeared to have a population of around 10,000 and, surprisingly, no social class distinction. Many individual homes have been identified - with several being reconstructed - and they are all similar. The living areas are laid out in a honeycomb-like pattern, with chambers being built below pedestrian roadways. There is no sign of an aristocracy, but communal kitchens and ovens are scattered throughout. A painting discovered at the site may in fact be the world's oldest map.

Also located in modern-day Turkey, Cappadocia was inhabited in the Hittite era, from 1,800 to 1,200 BC. Many structures have been found at this site, including burial tombs, living quarters and religious temples.

Tarxien Temple, part of Malta's ancient heritage.

7. Thailand: Phra Nakhon Si Ayutthaya

This city, originally built around 1,350 AD, includes monasteries and massive prangs, or reliquary towers. The site functioned as the second capital of Thailand, and was part of a sprawling and highly-organized urban plan that featured canals, roads, and interconnecting structures. The Burmese army sacked the city in the 18th century, but the remaining ruins tell the little-known story of a lost south Asian metropolis.

8. Italy: Monte d'Accodi, Sardinia

This Italian archeological marvel was discovered in 1954, and dates to 4,000 BC when the Ozieri culture prevailed in Sardinia. The main structure appears to have been used as a temple, altar, or observational pyramid, as there are no rooms laid out on or under its surface. Over 200 similar sites exist on Sardinia and the neighboring island of Corsica.

9. England: Stonehenge, Hadrian's Wall, and Roman Baths

Iconic as an unexplained prehistoric wonder, Stonehenge was built between 3,000 and 2,000 BC. Some speculate that it was a burial ground due to the human bones found there, while others believe that it functioned as an astronomical tool or a site of religious worship.

Hadrian's Wall, also referred to as the Roman Wall, was a Roman Imperial fortification in present-day England. It represented the northernmost border of the Roman Empire, running from the North Sea to the Irish Sea. It was 73 miles long, with a Roman fort every five miles. The wall's purpose was to protect the empire from the 'barbarians' to the north - mostly the Picts.

Built around 60 AD, the Roman Baths in the aptly named town of Bath, England, are incredibly well-preserved. Although no one is permitted to bathe in its waters, millions of visitors walk through the baths, which look very much today as they would have two thousand years ago.

Just a few of the Wats (temples) at Phra Nakhon Si Ayutthaya, Thailand.

10. Mexico: Chichen Itza

Built in the 7th century AD by the Mayans, the stepped pyramid at Chichen Itza is decorated with etchings of the Mayan snake god Kukulkan. Called El Castillo ("the castle") by the Spanish who conquered and colonized Mexico, this pyramid was once the center of a massive pre-Columbian city. Visited by millions, including those who vacation in nearby Cancun, access to the pyramids has been restricted in recent years due to concerns about damage and safety.

11. Denmark: Hulbjerg Jaettesue

The structure at Hulgjerg Jaettesue was built between 6,000 and 4,000 BC as a burial ground for the Funnelbeaker people of southern Denmark. This mausoleum was packed with the bones of 40 people, and the remains showed signs of fairly advanced dental practices, including root canals! The structure also includes tools, arrowheads, jewelry and other artifacts. And it was evident, too, that the ancient Danes visited their dead and possibly incorporated reverence of their remains into ceremonies.

12. Cambodia: Ta Prohm

This temple from the 12th century AD was built as a Buddhist monastery by the Khmer King Jayavarman VII. Since it is a relatively young ruin, it's actually in great shape, photogenic, with jungle trees and plants growing out of its walls. Famous photos of the site show tree roots draping down from its roof. In addition to being structurally intact, it contains a large number of bas-relief sculptures on its surface, such as praying figures and representations of the Buddha.

Chichen Itza, Mexico.

13. Morocco: Volubilis

A vital outpost of the Roman Empire in Northern Africa, Volubilis dates from the 3rd century BC. It remains one of the best-preserved examples of Roman architecture. Once home to an estimated 20,000 people, the city weathered the rise and fall of the Roman Empire, then flourishing against the backdrop of Christianity, Judaism, and Islam. When the city of Fez was established nearby in the middle ages, Volubilis started to lose its prominence, and its grand Roman features fell into disrepair. A massive earthquake in the 18th century almost leveled the entire city, but its former glory is still apparent even in its ruined state. Artwork, including well-preserved mosaics, still dot the site today.

14. Greece: Mycenae

The Mycenae civilization was comprised of neolithic to bronze age people who inhabited areas of Greece. Several of their construction projects survive from 1,600-1,100 BC. They range from modest stone houses to a large citadel. The still-standing ruins are some of the oldest in the world, and the Lion Gate is the only known monumental sculpture of bronze age Greece.

15. Australia: Burrup Penninsula

In 2016, archaeologists made an astonishing discovery off the northwest coast of Australia: evidence of human occupation over 9,000 years ago. This finding on the Burrup Penninsula is one of the oldest collections of ruins in the world. All that remains are the foundations of stone houses, which were built at the end of the last ice age.

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